1. Field of the Invention
The present invention relates to an extreme ultraviolet light source apparatus generating an extreme ultraviolet (EUV) light from a plasma generated by irradiating a target material with a laser light.
2. Description of the Related Art
In recent years, along with a progress in miniaturization of semiconductor device, miniaturization of transcription pattern used in photolithography in a semiconductor process has developed rapidly. In the next generation, microfabrication to the extent of 65 nm to 32 nm, or even to the extent of 30 nm and beyond will be required. Therefore, in order to comply with the demand of microfabrication to the extent of 30 nm and beyond, development of such exposure apparatus combining an extreme ultraviolet (EUV) light source for a wavelength of about 13 nm and a reduced projection reflective optics is expected.
As the EUV light source, there are three possible types, which are a laser produced plasma (LPP) light source using plasma generated by irradiating a target with a laser beam, a discharge produced plasma (DPP) light source using plasma generated by electrical discharge, and a synchrotron radiation (SR) light source using orbital radiant light. Among these light sources, the LPP light source has such advantages that luminance can be made extremely high as close to the black-body radiation because plasma density can be made higher compared with the DPP light source and the SR light source. Among these light sources, the LPP light source has such advantages that luminance can be made extremely high as close to the black-body radiation because plasma density can be made higher compared with the DPP light source and the SR light source. Furthermore, the LPP light source has such advantages that there is no construction such as electrode around a light source because the light source is a point light source with nearly isotropic angular distributions, and therefore extremely wide collecting solid angle can be acquired, and so on. Accordingly, the LPP light source having such advantages is expected as a light source for EUV lithography which requires more than several dozen to several hundred watt power.
In the EUV light source apparatus with the LPP system, firstly, a target material supplied inside a vacuum chamber is excited by irradiation with a laser light and thus be turned into plasma. Then, a light with various wavelength components including an EUV light is emitted from the generated plasma. Then, the EUV light source apparatus focuses the EUV light on a predetermined point by reflecting the EUV light using an EUV collector mirror which selectively reflects an EUV light with a desired wavelength, e.g. a 13.5 nm wavelength component. The reflected EUV light is inputted to an exposure apparatus. On a reflective surface of the EUV collector mirror, a multilayer coating (Mo/Si multilayer coating) with a structure in that thin coating of molybdenum (Mo) and thin coating of silicon (Si) are alternately stacked, for instance, is formed. The multilayer coating exhibits a high reflectance ratio (of about 60% to 70%) with respect to the EUV light with a 13.5 nm wavelength.
Here, as mentioned above, a plasma is generated by irradiating a target material with a laser light, and at the time of plasma generation, particles (debris) such as gaseous ion particles, neutral particles, and fine particles (such as metal cluster) which have failed to become plasma spring out from the plasma generation site to the surroundings. The debris are diffused and fly onto the surfaces of various optical elements such as an EUV collector mirror arranged in the vacuum chamber, focusing mirrors for focusing a laser light on a target, and other optical system for measuring an EUV light intensity, and so forth. When hitting the surfaces, fast ion debris with comparatively high energy erode the surface of optical elements and damage the reflective coating of the surfaces. As a result, the surfaces of the optical elements become a metal component, which is a target material. On the other hand, slow ion debris with comparatively low energy and neutral particle debris are deposited on the surfaces of optical elements. As a result, a compound layer made from the metallic target material and the material of the surface of the optical element is formed on the surface of the optical element. Damages to the reflective coating or formation of a compound layer on the surface of the optical element caused by such bombardment of debris decreases the reflectance ratio of the optical element and makes it unusable.
Japanese Patent Application Laid-open No. 2005-197456 discloses a technique for controlling ion debris flying from plasma using a magnetic field generated by a magnetic-field generator such as a superconductive magnetic body. According to the disclosed technique, a luminescence site of an EUV light is arranged within the magnetic field. Positively-charged ion debris flying from the plasma generated at the luminescence site are drifted and converge in the direction of magnetic field as if to wind around the magnetic line by Lorentz force of the magnetic field. This behavior prevents the deposition of debris on the surrounding optical elements, and thereby, the damages to the optical elements can be prevented. Additionally, the ion debris drifts while converging in the direction of the magnetic field. Therefore, it is possible to collect the ion debris efficiently by arranging an ion collection apparatus which collects ion debris in a direction parallel to the direction of magnetic field.
However, in the prior art, fast ion debris are supposed to collide with a collision surface of an ion collector device. This collision of fast ion debris sputters the collision surface whereby material of the collision surface flies out. Accordingly, there is a case where the sputtered material of the collision surface flies back again to the inside of the vacuum chamber and adheres to the optical elements such as the EUV collector mirror, and so forth, and an internal surface of the vacuum chamber.
On the other hand, if the target material adheres to the collision surface of the ion collector device, the adhered target material will be sputtered by the fast ion and fly out. As a result, there is a case where the sputtered target material flies back again to the inside of the vacuum chamber and adheres to the optical element such as the EUV collector mirror, and so forth, and the internal surface of the vacuum chamber.